1. Field of the Invention
The present invention relates to an electromechanical drive for actuating valves, especially in steam turbines.
2. Description of the Related Art
It is known to open and close valves in steam turbines with hydraulic or pneumatic drives, for example, to actuate an actuator for this purpose with a hydraulic cylinder or pneumatic cylinder in an electrohydraulic or electropneumatic manner. Examples for such actuators are described in German Patent documents DE 30 19 602 C2 and in DE 42 36846 C2.
Electrohydraulic or electropneumatic actuators are used for adjusting steam valves. Such actuators consist of a hydraulic cylinder or pneumatic cylinder which can be arranged in a single-acting or double-acting manner. A hydraulic/pneumatic control valve triggering the same is arranged as a 3/3 or 4/3 directional control valve and a position-sensing and a closed-loop control mechanism for the hydraulic cylinder or pneumatic cylinder. The position-sensing mechanism for the hydraulic cylinder or pneumatic cylinder, which can be arranged as an electronic position sensor or, in the simplest of cases, as a mechanical lever mechanism, is used to guide back the actual position of the hydraulic cylinder/pneumatic cylinder to the hydraulic/pneumatic control valve. A closed-loop position control loop for the hydraulic/pneumatic cylinder can thus be performed which allows a continuous setpoint-value-dependent positioning of the hydraulic/pneumatic cylinder.
One requirement in the adjustment of steam valves is that various fault conditions or events in the steam turbine require the steam valve to be closed reliably and very rapidly in order to bring the steam turbine to a secure state. Such quick-locking processes need to be performed even in the case of failure of the supply of medium of the hydraulic cylinder or pneumatic cylinder, e.g. the oil supply. This leads to the demand to enable performing the closing process independent of any functioning oil supply. Pressure springs are usually applied for this purpose to the actuator or the steam valve which exert the closing force independent of oil supply/air supply. Another solution is to provide the medium pressure reservoir as close as possible to the actuator and to remove the required medium quantity from the same and to exert the closing force.
For the feedback control of such actuators in steam valves in steam turbines, there are special requirements concerning the speed and the low-level signal behavior of such a feedback control. It is thus necessary that in the case of load changes in the steam turbine, as may occur, for example, in a sudden relief of a driven generator, the servo valve needs to move very rapidly to a new lower lifting position which corresponds to the new operating point of the generator and which holds the steam turbine within the permitted speed or frequency limits. If a demand for rapid closing occurs during such a feedback control process, it needs to be performed immediately. A precise low-hysteresis low-level signal behavior is required especially during the synchronization of a driven generator to the electric infeed network because no damping load acts on the steam turbine and thus small changes in the stroke of the actuator on the steam valve will lead to large changes in speed in the steam turbine. These high requirements can hardly be fulfilled with pneumatic drives. That is why they are only used in simpler applications such as steam turbines for compressor or pump drives. Electrohydraulically driven actuators are used otherwise.
The known actuators in steam turbines require a supply with a medium. Especially in the case of hydraulic drives a separate oil supply is usually necessary which supplies the required high medium pressure and oil quantity on the one hand and ensures the required oil purity which is required for operating directional control valves on the other hand. Moreover, complex mechanical piping is necessary which leads the medium to the actuator and from there back to the oil supply. There is always the likelihood of leakages of medium on the actuator and the piping, which is caused by improper piping or by the detachment of connecting elements by vibrations which are always present in steam turbines. These leakages of medium can lead to fires when flammable medium reaches the hot surface of the steam turbine. In order to avoid this, non-flammable medium liquids are used. However, they are expensive, lead to higher maintenance costs, and are hazardous for the environment which is why they have not prevailed as a solution to the problem. A further disadvantage of known actuator technology is that, in the case of electrohydraulic actuators, the mounting position cannot be chosen freely but is regularly predetermined in a fixed fashion, thus limiting the constructional freedom in the arrangement of such drives.
What is needed in the art is an electromechanical drive for actuating valves which avoids the disadvantages of the state of the art, especially by achieving a simple and secure actuation of valves of steam turbines.